The invention relates to a process for preparing a derivative of starch of a hydrophobic nature, as well as to a derivative obtainable in said process.
Hydrophobically modified biopolymers, such as pclysaccharides, find application in, inter alia, the textile and paper industries, in personal care products, bioplastics, water-based coatings, adhesives and inks. When dissolved or dispersed in water, these compounds possess favorable foaming and surface tension properties. Further, such compounds provide enhanced viscosification of aqueous solutions or dispersions containing the compounds.
It is believed that viscosity enhancement from increased hydrophobe content of such solutions or dispersions may be due to intermolecular association of individual polysaccharide molecules through the presence of hydrophobic groups or molecules to form polymeric networks. Support for this association has been observed in the rheology of the solutions. At relatively low shear rates up to 12 reciprocal secs, the solutions exhibit only mild pseudoplasticity at higher viscosities and near Newtonian behavior at lower viscosities. However, at shear rates of 200 reciprocal secs, the viscosity drops to very low values. This non-linear shear thinning behavior is consistent with the formation of intermolecular bridges by the presence of hydrophobic groups or molecules. Introduction of charged groups, which has conventionally been used for viscosity enhancement, has a number of disadvantages, such as sensitivity to the presence of salts. For instance, increased cationic substitution by quaternary nitrogen substituents which do not contain hydrophobic groups or molecules, causes a reduction in the solution viscosity at a constant high shear. It is believed that the increased cationic repulsion between molecules inhibits the formation of hydrophobe bridging resulting in a breakdown in the polymer network in solution.
Hydrophobically modified polymers, where the backbone is a cellulosic polymer, have been shown to possess an associative behavior. However, no associative behavior has been observed when the backbone is formed by starch. Järnström et al., in Nordic Pulp and Paper, Research journal, no. 3/1995, pp. 183–189, have described five different modified starches, of which three were hydrophobically modified by reaction with octenyl substituted succinic anhydride. They have observed that in relative concentrated solutions of these three hydrophobic starches, no associative behavior occurs.
The present invention aims to provide a hydrophobic starch which does show associative behavior. It is further an object of the invention to provide a hydrophobic starch which may be formed into a reversible gel. Yet another object of the invention is to provide a hydrophobic starch having excellent rheology regulating properties, such as a high viscosity when dissolved or dispersed in water.